What can cause the car to lose power when going forward but not in reverse? on 1999 Kia Sportage

We have replaced the fuel filter, fuel injectors, the coil packs, catalact convert,spark plugs, head gasket and for a short while that seemed to fix the problem. But now it is acting up again with the same issue as before. When you step on the gas to go forward, it has little to no power, but if you put in into reverse it runs fine.